Effect of nitric oxide synthase inhibition and nerve stimulation frequency on autonomous choroidal vasodilation

Abstract

Abstract Purpose To investigate the effect of non selective nitric oxide synthase inhibition and the influence of nerve stimulation frequencies on parasympathetic choroidal vasodilation. Methods Stimulations of parasympathetic nerve fibers of the greater petrosal nerve with 8 and 20 Hz were performed simultaneously with continuous measurements of intraocular pressure (IOP), choroidal blood flow (ChorBF), orbital venous pressure (OVP) and arterial pressure at the eye level (MAP). Stimulations were perfomed at baseline, after non selective inhibition of all nitric oxide synthases with L‐NAME and after systemic administration of atropine in an anesthetized acute rabbit model. Results Baseline values (mean ± sem, n=6): MAP 66.9 ± 1.8 mmHg, IOP 17.3 ± 0.8 mmHg, ChorBF 467 ± 54 P.U. Facial nerve stimulation with 8 Hz under baseline conditions causes a significant increase of ChorBF 660 ± 80 P.U. and IOP 20.7 ± 1.6 mmHg and a non significant decrease of MAP 61.1 ± 5.0 mmHg. The effects of 20 Hz stimulation did not differ significantly from the 8 Hz results (MAP 68.1 ± 5.0, IOP 22.86 ± 2.0, ChorBF 688 ± 73). Non selective inhibition of the nitric oxide synthases with L‐NAME (20 mg/kg) changed the vasodilatory effects of the two stimulation frequencies. The relative effect of 8 Hz stimulation was significantly smaller than the effect of 20 Hz stimulation. However, baseline blood flow after L‐NAME was lower than under baseline conditions. Conclusion 8Hz and 20 Hz parasympathetic stimulation had similar effects before but not after non selective NOS inhibition. However, no dose of L‐NAME completely suppressed the effect of parasympathetic stimulation.